The continuing decrease in the minimum feature size of modern complementary metal-oxide-semiconductor processes has necessitated a reduction in the circuit supply voltage for reliability and power-dissipation reasons. This reduction is problematic for traditional analog-to-digital conversion schemes because of the reduction in the resolution available within the amplitude range. On the other hand, the decrease in the minimum feature size has a beneficial effect on the obtainable time resolution in the circuitry, owing to the increase in the intrinsic speed of the transistors. Therefore, it is of interest to turn over the amplitude axis to the time axis and to encode information in the latter rather than the former (Roza, IEEE Transactions on Circuits and Systems—II: Analog and Digital Signal Processing, Vol. 44, No. 11, 1997). Such a concept is now known as “time encoding.”
Time encoding is a real-time, asynchronous mechanism for encoding the amplitude information of an analog band-limited signal into a time sequence, or time codes, based on which the signal can be reconstructed. Time codes can be generated by simple non-linear asynchronous analog circuits with low power consumption.
As described in Lazar and Tóth (IEEE Transactions on Circuits and Systems—I: Regular Papers, Vol. 51, No. 10, 2004), which is hereby incorporated by reference, time encoding of a band-limited function x(t) is a representation of x(t) as a sequence of strictly increasing times tk. Alternatively, the output of an encoder can be a digital signal that switches between two values ±b at times tk.
There are two natural requirements that a time-encoding mechanism has to satisfy (Lazar and Tóth, 2004). The first is that the encoding should be implemented as a real-time asynchronous circuit. Second, the encoding mechanism should be invertible, i.e., the amplitude information can be recovered from the time sequence with arbitrary accuracy.
Radio-frequency, or RF, signals are electromagnetic signals, i.e. waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation. Many communication systems modulate electromagnetic signals from baseband to higher frequencies for transmission, and subsequently demodulate those high frequencies back to their original frequency band when they reach the receiver. The original (or baseband) signal may be, for example, data, voice or video. These baseband signals may be produced by transducers such as microphones or video cameras, may be computer-generated, or may be transferred from an electronic storage device. In general, the high frequencies provide longer range and higher capacity channels than baseband signals, and because high-frequency signals can effectively propagate through the air, they can be used for wireless transmissions as well as hard-wired or wave-guided channels.
However, to our knowledge, there have been no successful attempts in the art at frequency conversion of time-encoded signals. There is a need in the art to provide circuits that are capable of being used as building blocks for software-programmable time-encoded radio receivers. What is essentially needed is a complete radio receiver system that includes effective time-encoded filters and down-converters.